The Sefer HaChinuch and the Symbolism of the 4 Minim

The holiday of Sukkot is filled with symbolism. There are countless explanations amongst the commentators, which speak of the true symbolism behind leaving our permanent homes for the temporary dwelling place of the sukkah. There are also many theories which detail the symbolic nature of the lulav, etrog, hadasim and aravot (the arbah minim).
One such explanation is offered by the Sefer HaChinuch. Throughout the Sefer Hachinuch the author develops the theory that,

אחרי הפעולות נמשכים הלבבות
The heart follows after actions

In essence the Chinuch feels that a person's being and psyche are influenced by their actions. If a person performs mitzvot, even if they are not initially motivated to do so, the performance of the mitzvah will leave an impression on them, eventually causing them to want to do mitzvot out of their own volition. This theory is very similar to the encouraging words of Rabbi Yehuda who said in the name of Rav (Horayot 10b),

לעולם יעסוק אדם בתורה ובמצות אפילו שלא לשמה, שמתוך שלא לשמה בא לשמה
A person should always be involved in Torah [study] and [the performance of] mitzvot even if it is not for their (the mitzvot's) own sake, because out of doing them not for their own sake, you will come to do them for their own sake.

The Chinuch (No. 285) writes that Sukkot is a time of great happiness for the Jewish people; having completed the harvesting of their crops at the beginning of autumn, the people would rejoice over the fruits of their labor. As a result God gave us a holiday, Sukkot, at this time, so that all celebrations should be done in His name, with the celebrants receiving reward for their performance of a mitzvah.
He goes on to explain, beautifully, the symbolism behind the arba minim,

ועוד יש בארבעה מינים אלו עניין אחר, שהם דומים לאברים היקרים שבאדם, שהאתרוג דומה ללב, שהוא משכן השכל, לרמז שיעבוד בוראו בשכלו, והלולב דומה לשדרה, שהיא העיקר שבאדם, לרמז שיישר כל גופו לעבודתו, ברוך הוא; וההדס דומה לעינים, לרמז שלא יתור אחרי עיניו ביום שמחת לבו; והערבה דומה לשפתים, שבהן יגמור האדם כל מעשהו בדיבור, לרמוז שישים רסן בפיו ויכוון דבריו ויירא מהשם אף בעת השמחה
These four species are similar to the vital limbs and organs of a person. The etrog is similar (in appearance) to the heart, the sanctuary of intellect, to indicate that a person should serve his Creator with his intellect. The lulav is similar (in appearance) to the spine, which is essential to man, to indicate that a person should straighten his entire body in His service, blessed is He. The hadas (myrtle) is similar (in appearance) to the eyes, to indicate that a person should not stray after their eyes on the day of his rejoicing. The aravah (willow) is similar (in appearance) to the lips, for with them a person completes all of his actions with speech, to indicate that a person should place a rein on his mouth and fear God even at a time of rejoicing.

The Chinuch, in keeping with his theory that we are influenced by our own actions, teaches that the purpose of the arbah minim is to make sure that we remain focused on God and maintain an even keel even during times of tremendous simcha.